The Serial Peripheral Interface (SPI) specification requires the transmitter to operate with both low data-dependent jitter (DDJ) and low transmitter waveform dispersion penalty (TWDPc, where the ‘c’ denotes direct-attach copper cable). SPI specification associated with Small Form Factor Pluggable (SFP+) specification compliant optical modules and copper twin axial cable, and 10GBASE-KR specification for backplane channels in computer servers and networking equipment are standards for 10 Gb/s serial data transfer.
The SFI TWDPc specification is associated with the copper twin axial cable channel and is defined as the ratio (in dB) between the signal-to-noise ratio (SNR) of a matched-filter receiver and the SNR at the slicer input of an ideal, adapted, FFE+DFE receiver. TWDPc measures the vertical eye openness (i.e., a voltage measure) while DDJ measures the horizontal timing jitter (i.e., a timing measure).
In data center applications, the same serial transmitter integrated circuit (with no change in transmitter programming) is used with both hot-pluggable twin axial cable and optical modules. Because of limiting laser drivers in optical modules, DDJ produced by the transmitter may not be equalized by the receiver at the far-end of an optical fiber link. Therefore, it is useful for the transmitter to satisfy both the SFI TWDPc and DDJ specifications at the same time. Satisfying SFI TWDPc and DDJ specifications for the same transmitter design is challenging because optimizing the transmitter design to meet TWDPc may make it harder to meet the DDJ specification.